Another take on Ardi’s pelvis

First, let me say that I agree with Zach’s assessment: the lordosis is completely imaginary, and I’m not at all convinced you can extrapolate it from a complete pelvis, let alone one with an imaginary sacral width.

Now, on to some more discussion of what the pelvis does (and does not) tell us:

Four questions that occurred to me while I read this article (in addition to the lordosis problems presented already by Zach):
1. Was Ardi bipedal based on the pelvis?
2. Wait, wasn’t Ardi a female? What about her birthing process?!
3. Was platypelloid the ancestral condition for hominid pelves?
4. How come there’s only four pages about the pelvis if it’s so darn complete? Have you seen how many pages were written on KNM-WT 15000? That pelvis was way less complete than this one!

Here’s my take:

The first question is by far the hardest (for me) to answer. Maybe? Sort of sometimes? I don’t even want to get into terrestrial vs arboreal bipedalism. I’m satisfied with saying she was capable of being bipedal but also of climbing (what type of climbing, I don’t dare try and suggest, I’ll just embarrass myself). Problems with saying Ardi is bipedal: the lower pelvis is totally chimp-like! How much could this affect locomotion? I don’t know. Most of the bipedal/not bipedal muscles would be attaching to the iliac blades, which are certainly more australopithecine-like than the lower pelvis. But that crazy curved medial portion of the ischio-pubic ramus has got to serve some functional purpose (unfortunately, for me to hazard a guess at this purpose, I would have to go study an anatomy text – something I’m just not going to do right now, so you’ll have to keep wondering). So maybe I can’t answer this question satisfactorily at this point (sadly the femur wasn’t a great help, and I don’t believe in the mythical lordosis).

I was told Ardi was a female (I believe this is mainly based on the skull, which I’ll admit I haven’t read about yet). HOW can you write a paper about a female primate’s pelvis and NOT discuss how she was having babies??? The only mention of this is at the very end of the pelvis section, where it’s pointed out that the differences we see between Ardi and Homo are probably due to “optimization of birth-canal geometry.” That’s lovely, but I want to know how Ardi gave birth – was it like a chimp or an australopithecine? How did that work with a mosaic pelvis? The birth canal seems awfully circular compared to Lucy, despite the flaring iliac blades that make it platypelloid. What does that mean? The lateral image of Lucy and Ardi shows that Ardi’s outlet was either gigantic (doubtful) or, because her sacrum would be positioned differently than Lucy’s, positioned antero-inferiorly compared to the australopithecine. Not to mention that the ischio-pubic region on Ardi would make for a much longer birth canal than what Lucy’s progeny had to deal with.

The platypelloid pelvis: Ardi is said to have one based on how the iliac blades were straightened out (something that could have been done differently, I’m sure). It was based on Lucy and Busidima (a Homo erectus), both of which were female hominids with platypelloid pelves. This is a great set up for this being the ancestral condition, and makes sense when we consider that a wider pelvis will biomechanically make bipedalism easier (human females have narrower pelves only because of obstetric constraints). So this is a great story. A word of caution though: we can’t assume Ardi’s pelvis is wide because she’s bipedal, AND that she’s bipedal because her pelvis is wide – that’s called circular logic! Assuming that her pelvis should be morphologically similar to other bipedal hominids in ways that make them bipedal, and then saying “ta da, Ardi is also bipedal!” doesn’t work.

This should have been a much longer paper. It’s ridiculous that the paper overall was so short, and that the pelvis had to share space with the (albeit almost non-existent) vertebrae and the femur. Get your own paper! Write more about the pelvis!! Address topics like obstetrics!!

This post refers to the same paper cited by Zach in the previous post – Lovejoy et al., 2009


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